Running palemoon-27.7.0a2_180108.linux-i686.tar.bz2 from the command line I noticed a warning had been triggered after I had closed PM, it looks like some regular 'this language feature has been updated and you should probably fix it in the next few years' sort of thing but I thought it might be helpful to pass on.

Yeah this is related to the way Mozilla changes things.. Pale Moon as an application does things in an older way that predates the change in the platform. Of course that change is related to e10s bs which we don't use.. As near as I can figure.. If we don't change that then when we run in e10s it won't work LOL.. Well we aren't Mozilla and we aren't gonna do e10s..

This appears only when opening a private window. I also did encounter this stupid warning many times and thought of reporting a few months ago. But knowing about http://xref.palemoon.org/ service, I decided to check it myself first. I was relieved when I found it was related to e10s crap which doesn't affect Pale Moon. Since then I've been ignoring these warnings.

OK, thanks for the explanation. Although interestingly when I quickly approximately recreated what I did earlier today from the history and memory I did not get the warning again. I used normal tabs in one window, visited sites I had used before, which were mainly text, no video or audio. I didn't open a private browsing window. So then I opened the Pale Moon forums in a private browsing window (a feature I never normally use), closed that, closed all the tabs and still didn't get the warning again. There is the possibility of user error I suppose, perhaps hitting CTRL instead of SHIFT at some point without noticing. But I don't know exactly what triggered it. Probably irrelevant though from your description.

Well in the time gap between the above posts I did follow the suggestion to reproduce the warning, but it came up slightly different, the first 2 warnings appeared when I went to the about:privatebrowsing page then the next 2 appeared when I clicked the Open a Private Window button.So my bug was a new irrelevant bug that finds another way to reference the unused e10s. And now I see what you are talking about with e10s - e(10 letters)s - https://wiki.mozilla.org/Electrolysis, so I guess at least I learned something new today.